You could be right Mr E, my water is well water so there is probably a big difference between that and tap water....you could add an entire bottle of ph down and it would only hold it for a day or two before crawling back up....
I do find that when I first do a res change it takes, as you say, very little but does not stay very stable for the first day or so until I have added more ph down etc, then it seems to stay pretty much where I put it about 48 hours in. Now flowering is a different story with the ph probs

I use baking soda as ph up usually as I only have to use small amounts and it's easier than ph up for me....I doubt I would use it if I had to raise the ph a few points or so....or have to use it often but I have not had any problems using it so far....keep it on hand with my other crowd of bottles. I have found it raises ph kind of slowly over a few minutes so I add a little and then check it a few mins later because the number seems to rise a little more after a min or two. I don't seem to have that particular situation when using ph up/down so just a a heads up.
Weedhound Reviewed by Weedhound on . PH Dropping quickly? I'm sorry to post this question, I tried to search but the term "PH" is too short for the boards search function... I'm having to PH Up my nutes more frequently as my plants grow. I'm wondering if this is considered normal, or if it is indicitive of any actual problems etc. I only use 8 to 10 gallons or so total (going to change it out and measure carefully today) and the plants are only about 10 inches tall, on their 8th node. I calibrated my continuous meter yesterday, and PH Up'd to Rating: 5